Characterizing the Variability of Boundary Currents and Ocean Heat Content Around New Zealand Using a Multi‐Decadal High‐Resolution Regional Ocean Model

Abstract

New Zealand is located in the southwest Pacific Ocean and is surrounded by a complex system of boundary currents that vary on a variety of time scales, with important impacts on weather, primary productivity, and fisheries. While various observational and modeling studies have shed light on many of the characteristics of New Zealand's ocean circulation, this study provides a comprehensive, quantitative, seamless, 3‐dimensional characterization of the region's boundary current circulation and ocean heat content. We use a 28 yr long hydrodynamic model simulation that accurately represents the mean and variability of the ocean circulation to characterize and quantify the temporal and spatial variability across the region. We show that low‐frequency variability in boundary current transport and upper ocean heat content are correlated over the entire New Zealand oceanic region. Oceanic eddies dominate heat content variability at intra‐annual scales in the northeast of the region, while on the west and southeast coasts heat content varies predominantly at interannual scales controlled by large‐scale changes in the subtropical and sub‐Antarctic fronts. The model shows a significant downstream strengthening and deepening of the boundary currents off northeast New Zealand which, if confirmed by observations, could suggest a significant new understanding of this Western Boundary Current system. This study presents a region‐wide characterization of the temporal and spatial variability of ocean currents and heat content, and their interconnections, providing a vital basis for understanding climate change impacts and the increasing occurrence of marine heatwaves.

Assessing the impact of subsurface temperature observations from fishing vessels on temperature and heat content estimates in shelf seas: a New Zealand case study using Observing System Simulation Experiments

“…In this section we discuss the differing processes at play across the NZ region in relation to the impact of the FVON and the data assimilation system, with implications for MHW predictability. The discussion draws on two previous studies that characterize the temporal and spatial scales of heat content variability around NZ (Kerry et al, 2023a) and reveal the varying drivers of MHW events across the regions .…”

“…The region provides an ideal test-bed as NZ experiences a complex system of boundary currents [Figure 1A; Chiswell et al (2015); Stevens et al (2019)] that modulate the surrounding oceanic environment on a variety of temporal and spatial scales. Both large-scale ocean currents and mesoscale eddies drive upper ocean heat content (UOHC) changes across the NZ region (Kerry et al, 2023a), driving MHWs with different characteristics depending on the local circulation region (Elzahaby et al, 2021;. Specifically, show that regional temperature extremes in coastal waters around NZ are largely driven by local circulation and highlight the importance of correctly representing the ocean's depth structure in predicting the onset of MHW events.…”

“…A thorough description of the model configuration and validation is presented in Azevedo Correia de . The model provides a realistic representation of the surface and subsurface variability around NZ, and represents NZ's major boundary currents well (Kerry et al, 2023a).…”

“…Spatial plots of the Root Mean Squared Difference (RMSD) between the Reference State and the comparison simulations are presented for the following metrics; SSH, SST, temperature at 50 m, temperature at 150 m, temperature at 400 m, temperature at 1000 m, bottom temperature, and upper ocean heat content (UOHC). The depth of 150 m represents the mean thermocline depth across the domain (Kerry et al, 2023a). The UOHC quantifies the heat carried in the upper ocean, and is given by:…”

“…where z is the height of the free-surface, −z T is the depth of the upper layer and C p is the specific heat of sea water in J(kgK) −1 . In this work we define the depth of the upper layer as the 90th percentile thermocline depth computed from a long-term hindcast simulation as in Kerry et al (2023a).…”